Overlay scrollbars expand only when mouse is near thumb

cc/layers/scrollbar_layer_impl_base.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_
 #define CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_
 
 #include "base/macros.h"
 #include "cc/cc_export.h"
 #include "cc/input/scrollbar.h"
@@ skipping 28 matching lines @@
   bool is_left_side_vertical_scrollbar() {
     return is_left_side_vertical_scrollbar_;
   }
 
   bool CanScrollOrientation() const;
 
   void PushPropertiesTo(LayerImpl* layer) override;
   ScrollbarLayerImplBase* ToScrollbarLayer() override;
 
   // Thumb quad rect in layer space.
-  virtual gfx::Rect ComputeThumbQuadRect() const;
+  gfx::Rect ComputeThumbQuadRect() const;
+  gfx::Rect ComputeExpandedThumbQuadRect() const;
 
   float thumb_thickness_scale_factor() {
     return thumb_thickness_scale_factor_;
   }
   bool SetThumbThicknessScaleFactor(float thumb_thickness_scale_factor);
 
   virtual int ThumbThickness() const = 0;
 
   // TODO(crbug.com/702832): No need for this function once there is element id
   // on overlay scrollbar layers.
   void SetOverlayScrollbarLayerOpacityAnimated(float opacity);
 
  protected:
   ScrollbarLayerImplBase(LayerTreeImpl* tree_impl,
                          int id,
                          ScrollbarOrientation orientation,
                          bool is_left_side_vertical_scrollbar,
                          bool is_overlay);
   ~ScrollbarLayerImplBase() override;
 
   virtual int ThumbLength() const = 0;
   virtual float TrackLength() const = 0;
   virtual int TrackStart() const = 0;
   // Indicates whether the thumb length can be changed without going back to the
   // main thread.
   virtual bool IsThumbResizable() const = 0;
 
  private:
+  gfx::Rect ComputeThumbQuadRectWithThumbThicknessScale(
+      float thumb_thickness_scale_factor) const;
+
   ElementId scroll_element_id_;
   bool is_overlay_scrollbar_;
 
   float thumb_thickness_scale_factor_;
   float current_pos_;
   float clip_layer_length_;
   float scroll_layer_length_;
   ScrollbarOrientation orientation_;
   bool is_left_side_vertical_scrollbar_;
 
   // Difference between the clip layer's height and the visible viewport
   // height (which may differ in the presence of top-controls hiding).
   float vertical_adjust_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarLayerImplBase);
 };
 
 typedef std::set<ScrollbarLayerImplBase*> ScrollbarSet;
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_